(1) Field of the Invention
The present invention relates to a feedback control circuit and LED driving circuit.
(2) Description of the Prior Art
In general, the driving methods for LED can be classified into a constant voltage driving and a constant current driving. Due to the characteristics of the LED, the constant current driving method can optimize the luminous efficiency of LED, and it is the most popular method of driving LED. The common constant current driving method uses an error amplifier to adjust a driving voltage of an LED string through detecting a negative end voltage of the LED string.
FIG. 1 is a schematic diagram of a conventional an LED driving circuit with constant current controlling. The LED driving circuit comprises a boost converter circuit, a control circuit 10 and a current control circuit ILC for driving an LED module LD. The boost converter circuit comprises an inductance L, a capacitance C, a diode D and a transistor M. One end of the inductance L is coupled to an input voltage Vin and the other end thereof is coupled to a positive end of the diode D. A negative end of the diode D is coupled to the capacitance C to provide an output voltage Vout for driving the LED module LD. The transistor M is coupled to a connected point of the diode D and the inductance L, and is switched according to a control signal Sdrv to make an electric power from the input voltage Vin be stored in the inductance L and the capacitance C. A positive end of the LED module LD is coupled to the output voltage Vout, and the negative end thereof is coupled to the current control circuit ILC. The current control circuit ILC controls the current flowing through the LED module LD at a predetermined current value steadily.
The control circuit 10 comprises an error amplifier 1, a compensation circuit 2, a PWM comparator 3, a logic circuit 4 and a driver circuit 5. An inverting input end of the error amplifier 1 is coupled to a negative end of the LED module L for receiving a detection signal IFB, and a non-inverting input end thereof receives a reference level Vr. An output end of the error amplifier 1 is coupled to the compensation circuit 2 and generates an error compensated signal Scomp at the compensation circuit 2 according to the detection signal IFB and the reference level Vr. A non-inverting input end of the PWM comparator 3 receives the error compensated signal Scomp, and an inverting input end thereof receives a ramp signal and accordingly generates a PWM signal Spwm. The logic circuit 4 receives the PWM signal Spwm and accordingly generates a PWM control signal Sct. The driver circuit 5 receives the PWM control signal Sct and accordingly generates the control signal Sdrv to control the duty cycle of the transistor M for adjusting the output voltage Vout.
FIG. 2 is a schematic diagram of another conventional LED driving circuit, adapted to drive the plural of the LED strings of the backlight module of the LCD monitor lighting. The currents of the plural of the LED strings L1˜LN are respectively controlled by current sources CS1˜CSN. A backlight control circuit 20 comprises a minimum voltage selection circuit 21, which is adapted to choose the minimum voltage among negative ends of all LED strings L1˜LN, and transmit a minimum voltage signal to an error amplifier 13. The error amplifier 13 controls a voltage supply circuit 11 according to the minimum voltage signal and a reference level Vr, and transforms an input voltage Vin into an output voltage Vout.
These control loop methods for driving LED require complicated loop compensation, which increases the difficulty in design.